In the manufacturing of color negative photographic printing papers, at least three light sensitive silver halide emulsion layers are used to capture the photographic image, i.e., red, green, and blue. These emulsions are placed in layers, and because of light scattering and absorption occurring in the upper layers, the lower layers receive less exposure.
The incandescent lamp normally used for exposing printing paper is low in energy output, particularly in a short wavelength region of the visible spectra. This reduces energy impinging on the layers of the photographic element. The color negative film to which the light is exposed onto the photographic printing paper often has masking dyes that give it a yellowish brown tint, further filtering the light and reducing the light available for exposure.
Nevertheless, there is a need to improve color reproduction of the original scene as captured in the color negative film. One way of achieving such an improvement is to employ more sensitive emulsions in the color print material. There exists a need to manufacture emulsions that have high sensitivity in order to overcome light deficiencies in exposure and capture the fidelity of the original color image.
Photofinishers also desire short processing times in order to increase the output of color prints. One way of increasing output is to increase the chloride content of emulsions, as chloride emulsions allow a higher development rate. Further, chloride ion in the development solution has less restraining action on development compared to bromide.
It has been recognized in the art that photographic sensitivity can be increased by adjusting the pH and/or the pAg of a silver halide emulsion.
Another way of attaining a high speed emulsion is through the use of reduction sensitization. Reducing agents such as stannous chloride, ascorbic acid, alkynylamino heterocycles (U.S. Pat. No. 5,399,479; U.S. Pat. No. 5,413,905; U.S. Pat. No. 5,500,333) and dimethylamine borane (U.S. Pat. No. 4,150,093 and U.S. Pat. No. 3,782,959) may be used. However, the use of such sensitizers may cause indiscriminate fog.
Thus there remains a need to achieve high speed silver halide photographic emulsion without the concommittant fog often associated with such speed increase.
Ripening agents have been used in photographic emulsions for making geometrically uniform grains. However, presence of ripening agents during precipitation can affect morphology of the grains. In general, the silver halide grains have rounded corners and edges, when precipitated in the presence of ripening agents. However, depending on the type of ripeners used, unwanted changes in the emulsion morphology may result. In addition, residual ripener adsorbed on the emulsion grain surface and not removed during washing and ultrafiltration may be carried over to subsequent emulsion preparation steps. Such remaining ripener may cause undesirable sensitization effects, and upon storage, deleterious fog. In the absence of ripening agents, silver halide grains produced have a narrower grain size distribution with sharper edges and corners.
Clearly, there exists a need to obtain emulsions with high sensitivity and narrow grain size distribution.
Recently, a new class of compounds has been reported to have benefits in stabilizing silver halide emulsions against such changes due to storage at high temperature and humidity. This class of heterocycle, in which one of the two sulfur atoms in a five-membered heterocyclic ring is oxidized to the tetravalent state (thiolone dioxide), has been disclosed in U.S. Pat. Nos. 5,693,460 and 5,670,307 and U.S. application Ser. No. 08/885,483 filed Jun. 30, 1997. U.S. Pat. No. 5,693,460 discloses the stabilizing properties on 3H-1,2-benzodithiol-3-one 1,1-dioxide. U.S. Pat. No. 5,670,307 discloses the combination of 3H-1,2-benzodithiol-3-one 1,1-dioxide with sulfinates. U.S. application Ser. No. 08/885,483, now U.S. Pat. No. 5,756,278, relates to the combination of water soluble gold sensitizers with thiolone dioxide compounds for enhanced emulsion sensitivity. U.S. Pat. No. 5,116,723 discusses the use of sulfur compounds for photographic use, and an example is provided in which a five-membered ring heterocycle contains three sulfur atoms.